1. Field of the Invention
The present invention relates to an apparatus for use in welding seams of thermoplastic resin sheeting that is heat weldable, and more particularly, to an apparatus for applying welds to an overlapping portion of two roof membranes one of which is fastened close to its edge by fastening means to an underlying roof structure for securely holding the welded membranes in place.
2. Reported Developments
Thermoplastic roofing material manufactured in sheets are designed to be laid down on the roof in a slightly overlapping manner or in an abutting relationship and weld together to form a watertight seal. If the overlapping method is used, typically, one to three inches of the overlapped sheets are welded together. The welding is accomplished by subjecting the overlapped portions to heat, softening them to a weldable consistency and pressing them together manually or by pressure wheels of an apparatus. When the overlapped portions of the sheets cool, a solid, watertight seal is formed at the seam of the welding. When the sheets are laid down with their edges being in an abutting relationship, the sheets are welded together by the use of a strip or tape of thermoplastic material laid along the butt which tape fuses to both sheets, and when pressed manually, or by pressure wheels of an apparatus, forms a watertight seal upon cooling.
When an apparatus is used to accomplish the weld, it is generally motor driven, self-propelled and automatic in which a drive motor is connected through a drive train to one or more drive wheels. A heating means, such as a hot-air blower, and a sheet-handling means are provided on the apparatus for guiding at least one of the opposing sheets through the apparatus and past the heating means as the apparatus travels along the edge of one of the sheets. The edges of the opposing sheets are heated and laid or placed together while hot. The hot sheets are then subjected to compression rolls or wheels and are allowed to cool.
The prior art is replete with various apparatus and methods designed and constructed with specific objectives in mind. Illustrative examples of the prior art include the following references.
U.S. Pat. No. 4,204,904 is directed to a roofing material handling and sealing machine which comprises a lifting foot which lifts the upper overlapping edge portion of a thermoplastic sheet so that a heating element may heat both thermoplastic sheets to soften them, after which the thermoplastic sheets are sealingly joined by a tailing rear wheel which presses them together.
U.S. Pat. No. 4,504,352 discloses an apparatus for sealing lapped joints of thermoplastic sheets having an interchangeable drive for driving either or both of a pair of drive wheels which allow the apparatus to move forwardly for traverse of the lapped joint in either of the opposite directions of travel. The apparatus also includes a heating element, a leading pressure roll and a traction connection which are all independently interchangeable between the left and right sides of the apparatus to permit reversal of operating direction, i.e. to seal either left-facing or right-facing lapped joints.
U.S. Pat. No. 4,855,004 is directed to an automatic seam welding machine for thermoplastic materials. The machine includes a heat gun gimbal mounted on the frame of the machine and comprises a blower, a heating element and a shaped foot to direct hot air between overlapped thermoplastic sheets. A thermocouple in the foot senses air temperature and is adapted to control power to the heating element.
The frame is mounted on three wheels, two of which are driven by a DC motor. One of the driving wheels is a wide pressure roller which rides over the seam of the thermoplastic materials to be welded to ensure a complete seal.
U.S. Pat. No. 4,872,941 is directed to an automatic welding apparatus which includes an electrically heated prismatic membrane, and a first and a second means to produce fist and second streams of heated air to heat the thermoplastic sheets to be sealed together.
U.S. Pat. No. 4,913,772 is directed to a portable thermoplastic welding machine that includes the improvement of being able to heat different and opposing thermoplastic sheets to different temperatures using independently controlled electric, radiant heat sources and then compressing the sheets with a compression belt.
The general state of the prior art may further be gleaned from the following U.S. Pat. Nos. 4,087,309, 4,146,419, 4,239,581, 4,289,552, 4,354,893, 4,259,142, 4,447,288, 4,533,423, 4,725,328, 4,737,213, 4,744,855, 4,861,412, 4,894,112, 5,234,533, 5,250,143, 5,378,302, 5,569,352, 5,624,511, 5,776,299.
In laying thermoplastic sheets in overlapping relationship each other, it is not sufficient to fuse the overlapping portions of the sheets to each other. The sheets must also be secured to the underlying roof deck which holds the fused sheets so that environmental conditions, such as high wind, do not separate the fused sheets from the roof structure. For this reason a fastening mean is used to firmly hold the sheets on top of the roof deck. In a typical process, which takes place at the job site, a first thermoplastic sheet, obtained in a roll form from a supplier, is laid on a portion of the deck starting from one side of the roof. Then a fastening means, such as a batten bar or a line of stress plates is placed close and parallel to the edge of the sheet. The batten bar or line of stress plates on top of the sheet is secured to the underlying deck by securing means, such as screws or nails. Typically, the batten bar or stress plates are provided with holes through which the nails and screws can be driven into the deck. After this step in the process a second thermoplastic sheet is laid on the roof deck parallel to the first thermoplastic sheet and in an overhanging relationship therewith wherein the overlapping portion covers the fastening means on both sides thereof. A weld is then applied between the overlapped portion of the first sheet and the overlapping portion of the second sheet on one side of the fastening means and then another weld is applied between the overlapping portions of the sheets on the other side of the fastening means. As stated in U.S. Pat. No.4,834,828, one of the disadvantages of this process is the necessity of having to apply a weld on one side of the fastening means and then apply a second weld on the other side of the fastening means. Said patent provides an improvement over the prior art by providing welding means that is capable of applying welds to thermoplastic sheets simultaneously on both sides of the fastening means. The apparatus to accomplish this objective includes a weld wheel comprising three portions: a first portion capable of being positioned on one side of the fastening means; a second portion capable of being positioned on the other side of the fastening means; and a third portion being a groove between the first and second portions designed to be positioned over the fastening means. First and second portions of the weld wheel apply pressure on the sheets on opposite sides of the fastening means, while the groove rides over on top of the fastening means.
As well known in the prior art, fastening means rise above the horizontal level of the first thermoplastic sheet laid down on the roof deck and, consequently, hold the second overlapping portion of the second sheet above the horizontal. If the fastening means are bulky, the rise above the horizontal level is accentuated. In addition stress plates, positioned spaced from each other in a line parallel to the edge of the sheet, form hills and valleys resembling a sinusoid or side curve which does not provide a smooth ride for the groove between the first and second portions of the weld wheel. The uneven ride of the groove results in uneven pressure exerted on the underlying thermoplastic sheets: when the groove rides over a stress plate, the pressure is greater, while the pressure in the space between the plates is less. The uneven pressure results in air pockets or bubbles as a result of hot air being entrapped between the sheets. Varying the dimensions of the groove does not eliminate the problem: if the depth of the groove is such that its bottom surface barely touches the fastening means, larger air pockets or bubbles are formed. On the other hand, if the depth of the groove is shallow, the first and second portions of the weld wheel tend to be lifted upward when the groove passes over the stress plates thereby reducing the required pressure of the weld wheel to press the overlapping layers together.
A main object of the present invention is to provide a weld wheel the use of which prevents the formation of air pockets or bubbles over the fastening means.
In accordance with the present invention there is provided a pressure roller and a heating element for a welding apparatus for producing a weld simultaneously on each side of a line of fasteners and over the fasteners. The pressure roller is integral with an axle designed to be connected to a driving means at one end thereof; at the other end of the axle the pressure roller comprises a distal end, a proximal end and a center portion which defines a groove between the proximal and distal ends. The groove of the pressure roller carries an elastomeric cushion designed to smoothly ride over a line of fastening means.
The pressure roller is used in combination with a heating element containing a blower which forces heated air through a nozzle. The nozzle having an outlet therein comprises three portions: two large opening portions and a restricted opening portion therebetween. In use, the large opening portions allow delivery of the major portion of the hot air produced by the heating element while the restricted opening still allows delivery of sufficient amounts of the heated air to soften the overlapping portions of the thermoplastic sheet over and under the fastening means.